a(n) = 13*(2^n - 1) - 3*n^2 - 9*n.

Terms

a(0) =1a(1) =9a(2) =37a(3) =111a(4) =283a(5) =657a(6) =1441a(7) =3051a(8) =6319a(9) =12909a(10) =26149a(11) =52695a(12) =105859a(13) =212265a(14) =425161a(15) =851043a(16) =1702903a(17) =3406725a(18) =6814477a(19) =13630095a(20) =27261451a(21) =54524289a(22) =109050097a(23) =218101851a(24) =436205503a(25) =872412957a(26) =1744828021a(27) =3489658311a(28) =6979319059a(29) =13958640729